An Islamic State militant has murdered his mother in public in the Syrian city of Raqqa, according to human rights groups.

The woman, in her 40s, had reportedly warned her son that a US-backed alliance would wipe out Isis and had encouraged him to leave the city with her.

She was detained after he informed the group of her comments, according to the Syrian Observatory for Human Rights, a British-based group that monitors the war through a network of sources on the ground.

Citing local sources, the Observatory said the 20-year-old man killed his mother on Wednesday near the post office building where she worked in front of hundreds of people in Raqqa, a main base of operations for the group in Syria.

According to a local resident, the assailant’s mother was originally from Jabla, a town in Latakia that is a regime stronghold. She was Alawite, but married a Raqqa man who divorced her about five years ago. She stayed in Raqqa with her son and daughter, now 20 and 25.

The resident said Isis told people she was killed for apostasy. “This was the first time someone has executed his own mother,” he said. “People are shocked that someone can kill his own mother in such cold blood. He was known to be a bad apple but nobody imagined he would go so far as to kill his own mother. Everybody is asking how they could have brainwashed him so much?”

Raqqa is Being Slaughtered Silently, an activist group documenting atrocities committed by Isis and the Assad regime, also reported the murder on social media, naming the killer as Ali Saqr.Isis, which controls large swaths of Syria and Iraq, has killed hundreds of people it has accused of working with its enemies or breaching its ultra-conservative interpretation of Islam.

The Observatory reported on 29 December that Isis had killed more than 2,000 Syrian civilians in the 18 months since it declared a “caliphate” over the territory it controls in Syria and Iraq. They included people killed on the grounds of homosexuality, practising magic and apostasy.